Filter
Top Products
4-16 IBM Informix OnLine Database Server Administrator’s Guide
Processing
The reason for the delay is to ensure a proper restore. The level-0 archive
copies the updated OnLine configuration information (the newly created
mirror chunk) from the root dbspace reserved pages to the first block of the
archive tape. In the event of a data restore, the updated configuration infor-
mation at the beginning of the archive tape can direct OnLine to look for the
mirrored copies of the logical log files if the primary chunk becomes
unavailable. If you did not have the new archive information at the beginning
of the tape, OnLine would be unable to take advantage of the mirrored log
files.
This is also the reason why you cannot mirror a dbspace that contains a
logical log while an archive is being created. The new information that must
appear in the first block of the archive tape cannot be copied there once the
archive has begun.
Processing
During OnLine processing, mirroring is performed by executing two writes
(one to the primary chunk and one to the mirror chunk) for each modified
page.
OnLine detects an I/O error by checking the return code when it first opens
a chunk and after any read or write. If OnLine detects that a primary chunk
device failed during a read, OnLine changes the chunk status to down (D) and
begins reading from the mirror chunk. OnLine continues to read from the
mirror chunk for as long as the primary chunk status remains down.
If OnLine detects that a primary chunk device failed during a write, writes
continue to the one chunk that remains online. This is also true if one of the
chunks is intentionally brought down by the administrator. Writes continue
on the other chunk.
Once the down chunk is recovered and returned to online status, reads are
again performed on the primary chunk and writes are made to both the
primary and mirror chunks.